Cut four 15" squares of aluminum foil or parchment paper. Fold each square in half. Cut a large heart from each folded paper. Lightly brush center area of each heart with oil.

Place one fish fillet on one-half of each heart. Lightly paint each fillet with mustard. Distribute mushrooms and onions over fish. Sprinkle with seasonings. Re-fold heart along crease, covering fish. Fold cut edges together to seal and make a tightly-sealed packet.

Stack packets on rack in Nesco® Roaster Oven. Cover and cook at 400ºF for 45 to 60 minutes, or until fish flakes easily with a fork.

To serve, place a packet on each plate. Split to open. (Use caution as steam will be hot.)